An example of a differential amplifier used as a multiplier circuit is described in U.S. Pat. No. 4,156,283 issued May 22, 1979 to Gilbert. Two signals are multiplied in this circuit. First, a pair of control signals are applied to a first differential amplifier for producing a pair of differential signals from an input signal. These signals are further modulated by a second pair of control signals. This second pair of control signals is generated by a second differential amplifier. There is therefore produced a pair of output signals the difference between which represent a multiplication of the two sets of control signals.
Conventional differential amplifier controllers, such as the one disclosed by Gilbert tend to be sensitive to power supply variations, are somewhat complex, and use a significant amount of power.
The present invention provides a controller for a differential multiplier which uses less power, is simpler in structure, and relatively insensitive to power supply variations. In addition, because the gain is dependent only upon currents I.sub.1 and I.sub.2, the gain accuracy of the circuit is much improved over standard Gilbert multiplier implementations.
A differential amplifier is responsive to first and second control signals for generating from an input signal first and second output signals, respectively. The controller of the present invention generates a digital signal representative of a desired relationship between the first and second output signals. The digital signal is converted into the first and second control signals such that the desired relationship between the first and second output signals results from applying the control signals to the differential amplifier.
In the preferred embodiment, the digital signal is converted by a digital to analog converter (DAC) for converting the digital signal into a pair of analog signals. The DAC outputs the analog signals as currents on a pair of conductors, with the sum of the currents being a constant and the relative magnitudes of the currents being determined by the digital signal. Means are provided for maintaining the voltages of the conductors carrying the analog signals at a common level for preferred operation of the DAC.
Differential amplifiers are useful in a variety of situations which require amplification or multiplication of signals. The present invention was developed for manually controlling a signal input to an oscilloscope. For each knob position a unique digital signal is generated for controlling the scope image. Appropriate knob positions also provide for inverting the signal. Thus, the viewed signal image can take a range of .+-.1 times the input signal. It will be appreciated however that the invention has applicability to a variety of applications.
Thus a simple controller is provided which is easy to operate, dependable and reliable. These and other features and advantages of the present invention will become apparent from a reading of the following detailed description of the preferred embodiment in conjunction with the accompanying drawings.